Turbulator for liquid cooling system for computers

ABSTRACT

A turbulator for use in cooling computer systems is disclosed. The turbulator is disposed in a heat exchanger tube and is configured to force the fluid in a path with length more than twice the largest dimension of said heat exchanger tube. By increasing the path and thus the surface area of the heat exchanger in contact with the fluid, and by causing the fluid to swirl in the heat exchanger tube, the turbulator achieves a higher heat dissipation efficiency for the computer cooling system.

BACKGROUND OF THE DISCLOSURE

Arrays of electronic computers, such as are found in data centers,generate a great deal of heat. Liquid cooling is recognized as anexcellent way to cool computer CPUs due to the high concentration ofpower, but the rest of the electronics puts out heat at a lower leveland at a lower intensity, so air-cooling is appropriate for much of theassociated hardware. Current systems may use liquid cooling the move theheat from the CPU to a radiator mounted close to the CPU, or they mayuse an air to liquid heat exchanger to remove heat from the computerenclosure and heat up liquid in the heat exchangers. These systemssuffer from the high thermal resistance and bulkiness of liquid to airheat exchangers. Other systems use a chilled water loop to cool thecomputer, but these systems require complex and expensive connectors andplumbing to connect the server to the building water supply whileinsuring that no leaks occur. Furthermore, chillers require a largeamount of power. Since the air-cooling system is already required,air-cooling should be used as a backup for the water-cooling for the CPUand other heat producing components. For operation in a data center,servers, particularly blade servers, need to be compact. Therefore, anideal CPU heat exchanger would be compact, would use water-cooling asthe primary means of cooling with air-cooling as a backup system.Furthermore, the expense and difficulty of servicing the plumbing systemwould be reduced if the system ran at less than atmospheric pressure, sothat leaks are of air into the cooling system, instead of liquid intothe data center. Therefore an ideal CPU cooling system would use liquidas the primary means of cooling, with air backup and both the air andliquid heat exchangers would be coupled to the CPU. Furthermore, if theliquid cooling system is operational, the air cooled portion of the heatexchanger may be used to remove heat from the air inside the serverenclosure, thereby using the CPU heat exchanger as a way to remove heatfrom the server and the data center environment.

Therefore it is an object of the present invention to provide anefficient and compact liquid heat exchanger for a CPU under negativepressure, with a minimal chance of leakage, and with an air-coolingbackup system. It is also an object of the present invention to providea cooling solution which integrates with an air-cooled heat sink forbackup and utilizes only the minimum amount of water to provide adequatecooling for each heat-generating element. Furthermore, it is an objectof the present invention to provide a way to cool the CPU, the serverand the data center with liquid in an optimal manner, cooling the CPU toreduce the leakage current, removing heat for the data center in generalby means of the air cooled portion of the CPU heat exchanger, andutilizing an outdoor evaporative cooling system so as to eliminate theneed for a chiller in the liquid cooling system.

SUMMARY OF THE DISCLOSURE

A liquid cooled heat exchanger utilizes a turbulator in a tube toenhance the heat transfer from the liquid to a heat-generatingcomponent. It does this by reducing the cross sectional area andincreasing the length of the flow path through said tube. This may bedone, for example, by placing a threaded rod in a tube with a smalldiametrical clearance. The heat exchanger is designed to take up theminimum amount of space, while providing maximum heat transfer, andrequiring only a modest pressure drop. The turbulator itself displacesmuch of the volume inside the heat exchanger, reducing the amount ofcoolant inside the electronics environment. In addition the system mayinclude an air-cooled heat exchanger attached to each CPU to remove theheat in the event that the liquid cooling system is not operating.

The water-cooled heat exchanger may be mounted to the CPU and comprisesa passage with a turbulator to increase the velocity and turbulence ofthe water near the heat transfer surface. The turbulator may also bedesigned to minimize the volume of water contained within the server sothat the water may be quickly cleared for repairs. The CPU typicallyincludes an air-cooled heat exchanger with fins and a fan located nearbyto provide air-cooling. The temperature of the CPU may control the fanso that as it gets hotter, the fan increases in speed. The liquid flowrate may be determined by the acceptable temperature rise of the liquidand the power dissipated by the CPU.

For a typical CPU that puts out 100 watts, a stream of water at 150cc/minute will result in a temperature rise of approximately 10 C. Atypical air cooled heat exchanger may have a thermal resistance of 0.15C/watt. The liquid cooled heat exchanger may have a thermal resistanceof 0.05 C/watt. By adjusting the position of the water-cooled heatexchanger within the assembly the thermal resistance from the air to thewater and the

CPU may be suitably controlled so as to provide optimal cooling for theair in the data center and the CPU chip. In some cases, multiplepassages may be used to cool both the fins and the processor. Thetemperature differential from the CPU case to the water should be of thesame order as the temperature rise of the liquid as it flows through theheat exchanger. The heat exchanger should have a pressure drop ofapproximately 4 in Hg so that the system will work properly on a hot dayin a high altitude location, where the difference between the localatmospheric pressure and the vapor pressure of the hot water can be only8 inches Hg. The remainder of the pressure drop can be used for plumbingto and from the heat exchanger and the pump, including head loss,elevation changes, and increases in flow resistance due to fouling.

The fan which is connected to the CPU heat exchanger may also be used tocool the interior of the computer by transferring heat from the airinside the computer to the water so that other components within theserver enclosure may be cooled with or without the use of external airflow—i.e., the computer may be sealed. The speed of the fan may beadjusted to remove additional heat from the air inside the serverenclosure of the data center as required to minimize the overall powerconsumption of the data center. The overall power consumption versus fanspeed may be determined based on the power consumption of the airconditioning system vs. temperature in the data center and the powerconsumption of the CPU vs. temperature. CMOS based processors use moreenergy as the temperature of the processor goes up, due to leakagecurrents. The air conditioning system uses additional power depending onthe temperature of the data center and the heat removal. This increaseis generally linear; with higher temperature require proportionallyhigher air conditioning power. The CPU uses additional power dependingon the temperature of the processor due to leakage currents, with theleakage currents increasing exponentially as the processor at the higherend of the temperature. By controlling the flow rate of liquid and airthrough the heat exchanger, and by adjusting the position of the liquidheat exchanger in the overall assembly consisting of a base and fins,the overall power required for the data center can be decreased. Thisflow of heat may be suitably analyzed using an electrical analog asshown in FIG. 7.

The water reservoir, which supplies water to the heat exchanger,preferably has biocidal and anticorrosive agents to prevent fouling ofthe heat exchanger.

The plumbing to and from the server may be designed for a low pressuredrop so that the majority of the pressure drop occurs through the heatexchanger. In general, air will be dissolved in the liquid, so that asthe liquid is subjected to low pressure and heat, some of the air willcome out of the liquid. The system must be designed to pump this out andthe plumbing from the heat exchanger may be of larger cross sectionalarea than the plumbing to the heat exchanger. This has the added benefitof reducing the chance for misconnected plumbing.

Each server may be fitted with pressure regulator to control thepressure drop across the server, depending on it cooling requirementsand a filter may be used after the cooling tower and before the heatexchanger to prevent clogging of the heat exchanger passages. Liquidcoolant with chemical additives may be used to prevent fouling of theheat exchanger with biological films and to prevent corrosion. Theinternal heat exchanger passages may be plated or anodized to preventcorrosion.

A vacuum reservoir may be located at each server rack, and it may have afloat actuated air release to allow for the release of any accumulationof air. Vacuum lines may connect back to a centrally located vacuumsink.

Each server or server rack may be connected with a dry disconnect systemthat allows for the automatic draining of the server system. Theconnector may utilize a sacrificial metal, such as zinc or utilizeelectrical potential to prevent corrosion inside the CPU heat exchanger.Water would be the best heat exchange fluid due to its low viscosity andhigh heat capacity. Perfluorocarbons or avionics cooling fluids may alsobe used. Tap water which has a slight alkaline content that may reducethe corrosion rate for copper and brass heat exchangers.

The heat exchanger may use a helical flow pattern to put a long pathinto a short passage. This helical flow passage may have multiplestarts, so as to allow for increased flow in a small passage. This maybe accomplished by placing a threaded rod in a metal tube so that theflow must take a long path through the heat exchanger at a highvelocity. This has the added benefit of reducing the volume of water inthe heat exchanger, thereby reducing the amount of water that needs tobe cleared to service the heat exchanger. Alternatively, a rod with atortuous path in relief may be used to displace fluid in the center partof the passage and thereby increase the water flow and turbulence.

The rod and cylinder may be square, cylindrical, conical, triangular,hexagonal, or any other appropriate shape. The turbulator may bedesigned so that some of the water flows over the flow passages in anaxial direction. This axial flow will interact with the helical flow toprovide swirl in the heat transfer passages in order to increase heattransfer. In addition, the axial flow will reduce the flow resistance ofthe heat exchanger. This arrangement may be particularly useful insituations where the flow is laminar or nearly laminar. For high powerdissipation systems, or for additional reliability, multiple parallelturbulators may be used.

Although a CPU is described, this system maybe used to cool anyelectronic component. Although water is described, any coolant may beused instead of or in addition to water. Although the system isdescribed as using water for evaporation and for cooling, a liquid toliquid heat exchanger may be used to transfer heat from an evaporativesystem to a closed system so that a non-corrosive or non-conductivecoolant may be used for the CPUs. This may be used in the case ofevaporative coolers which use salt water or reclaimed water, forexample. For low temperature operation, as in

Northern latitudes, a radiator, fan and glycol system may be used toreject the heat and prevent freezing of the coolant. Since CPUs can getup to 60-70 C, water can be heated to 50 C and used for hot waterservice. The water used for cooling the computers may be kept at atemperature higher than the dew point of the air in the data center toprevent condensation on the plumbing or the heat exchangers.

Any leakage in the system may be detected by monitoring the cycle timeof a pump used to remove air from the system. If the pump is cycling ontoo often, then a leak is indicated. The leak may be discovered bypulling a vacuum on each server and measuring the decrease in vacuumover time. A simple hand operated vacuum pump may be used for this typeof testing.

The system may use a pump with a reservoir to supply fluid to all theheat exchangers. During a shutdown procedure, the pump may evacuate thesystem; purge it with air and store the fluid until such time as theliquid cooling system is reactivated. During a reactivation procedure,the pump control system may apply a vacuum or a pressure to the system,check to see if the fluid system loses vacuum and then start pumpingagain, based on the rate of change of the system pressure.

The coolant used for the computers may be separate from the cooling usedfor other systems. In this case the heat can be transferred from onesystem to another using a plate type heat exchanger in a separatecooling loop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of turbulator and flow passage with arectangular cross section and a single entry flow passage.

FIG. 1B is a top view of turbulator with a rectangular cross section anda single entry flow passage.

FIG. 1C is a side view of turbulator with a rectangular cross sectionand a single entry flow passage.

FIG. 2 is a top view of turbulator with a rectangular cross section anda double entry flow passage.

FIG. 3 is a side view of turbulator with a circular cross section and asingle entry flow passage.

FIG. 4A is an isometric exploded view of a typical air and water cooledheat exchanger with turbulator near the heat source

FIG. 4B shows how the liquid coolant may be connected to the heat sinkwithout changing the footprint of the heat sink by removing a few finsand adding a fluid connection.

FIG. 5 is a side view of a typical air and water cooled heat exchangerwith turbulator near the fins.

FIG. 6 is a section view of a typical air and water cooled heatexchanger with turbulators in the fins.

FIG. 7 is a diagram showing the heat flow in a server environment whichuses a liquid and air cooled heat sink.

FIG. 8 is a perspective view of a turbulator with a circular crosssection and a double entry flow passage.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The following detailed description presents a description of certainexample embodiments of the present invention. In this description,reference is made to the drawings wherein like parts are designated withlike numerals throughout.

Referring to the example embodiment shown in FIGS. 1A, 1B and 1C, aturbulator 1 is made with a helical flow passage which forces the flowto go diagonally across one face of the interior of a rectangular heatexchange tube 2 and then across to the other side of said passage, wherethe flow goes diagonally across the and then back to the previous side.The flow passage is fed with a fitting 3, which may include a hose barb.

Referring to FIG. 2, an alternate turbulator forms a double entryhelical flow path. This design allow for more flow at a given pressurethan the design in FIG. 1, in that is uses two parallel flow paths. Theuse of two paths, instead of one larger, but rectangular path, increasesthe velocity of the fluid and makes the device resistant to clogging.Also, it reduces the tendency of the flow to short circuit, as in thecase of a larger but rectangular flow path, wherein the flow goesdirectly from one corner to the other and leaves areas of flowrecirculation.

Referring to FIG. 3, in this case a circular turbulator is used insideof a circular tube. This may be easily constructed by placing a threadedrod in a tube with a close tolerance. This type of design lends itselfto use in some of the embodiment described below, in which the heatexchanger is embedded in the fins in order to reduce the thermalresistance to the air.

Referring to FIG. 4A, in this embodiment, the heat exchanger tube 2 issoldered into a slot in the base plate 32 of the heat sink 30, therebyreducing the thermal resistance from the CPU to the liquid. Theturbulator 1 enhances the heat transfer from the liquid to the base ofthe heat sink and to the top of the CPU (not shown).

Referring to FIG. 4B, a fitting, 31 is used to connect to a fluid supplyand return system so that the cooling system can be connected withoutaffecting the mechanical attachment of the heat sink to the CPU orcircuit board. The path of the heat exchange tubes is shown by dashedline 33.

Referring to FIG. 5, in this case the heat exchanger tube 2 ispositioned on the top of the heat sink base plate 32 . In thisconfiguration, the thermal resistance from the CPU to the liquid coolantgreater than is less than in the design in FIG. 4A and B However, thethermal resistance from the liquid to the air is reduced.

Referring to FIG. 6, the heat exchanger tubes 2 are placed away from thebase plate, 32 in order to further reduce the thermal resistance fromthe liquid coolant to the air. A manifold, 41, is used to distribute thecoolant to the two heat exchanger tubes 42 and 43. The distance from thebase plate to of the tubes may be adjusted in order to adjust thethermal resistance from the liquid coolant to the air and from the CPUto the liquid coolant.

Referring to FIG. 7 a thermal model of the system is shown. In thismodel, the thermal resistance from the CPU to the liquid and the air,and from the air to the liquid is illustrated. By means of the priorembodiments, the thermal resistance from the heat sink to air, the CPUto the heat sink, the heat sink to the liquid and the air to the liquidmay be adjusted to minimize the overall power consumption of the datacenter. For example, increasing the number or area of the fins, which iswell known in the art, may decrease the thermal resistance from the heatsink to the air. The thermal resistance from the air to the liquid maybe decreased by placing the liquid heat exchanger closer to the centerof the fins. Heat pipes may also be used to control the flow of heat.

FIG. 8 illustrates yet another type of turbulator with a circular crosssection. The turbulator forms a double entry helical flow path. Toillustrate these paths,

FIG. 8 has a black shading 50 that illustrates one path, while thenon-shaded area 52 illustrates the independent second path. This designallow for more flow at a given pressure than the design in FIG. 1, inthat is uses two parallel flow paths. The use of two paths, instead ofone larger path increases the velocity of the fluid and makes the deviceresistant to clogging. Also, the dual path and circular cross sectionreduces the tendency of the flow to short circuit, thus maintaining theflow in thermal contact with the heat exchange tube and increasingcooling efficiency.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent invention as would be understood to those in the art asequivalent and the scope and context of the present invention is to beinterpreted as including such equivalents and construed in accordancewith the claims appended hereto.

1. A liquid cooling system for cooling an electrical device, comprising:a fluid-containing heat exchanger tube thermally coupled to saidelectrical device; a turbulator within said heat exchanger tube; saidturbulator configured to force the fluid in a path with length more thantwice the largest dimension of said heat exchanger tube. wherein saidturbulator reduces a flow path cross sectional area to less than 50% ofthe cross sectional area of the original tube.